in reference to an unsuspecting asian person (esp. a tool), used by a non-asian solely as a way to meet other asians and to infiltrate their culture.
1) That American girl has never met an asian guy before. So she befriends the naive Takeshi -- her gateway asian. Now that she is comfortable with being around Japanese people and is seen by enough of his countrymen, she abandons Takeshi, eats a lot of sushi, and sleeps with hot asian guys.
2) Japanese kid (to an American girl): stay the fuck away from me. I'm not your gateway asian.